Fiber adhesive

ABSTRACT

Tacky polymeric fibers having a nontacky core and a tacky outer shell.

The invention relates to a tacky polymeric fiber having a nontacky core and a tacky outer shell.

In the case of adhesives, and particularly in the case of pressure-sensitive adhesives as well, there is a desire both for effective cling to the substrate to be bonded (good adhesion) and for a sufficient internal strength within the adhesive layer (cohesion). Adhesion and cohesion are mutually opposed performance properties. Measures which produce an improvement in adhesion generally lead to a deterioration in cohesion.

New adhesives or adhesive systems ought as far as possible to have better performance properties than the existing adhesives, and in particular both the adhesion and the cohesion ought to be good.

In addition to this there is a desire for adhesive systems having new performance properties. For adhesives of such kind there are then, correspondingly, completely new application possibilities available.

An object of the invention, therefore, were adhesive systems having improved and innovative properties.

In accordance with this object the polymeric fibers defined at the outset have been found. Also found have been processes for producing and for using these polymeric fibers.

The polymeric fiber of the invention is composed essentially of a nontacky core and of a tacky outer shell.

The Core

The core comprises a polymer (referred to below for short as core polymer) which is thermoplastically deformable.

This polymer is in particular a thermoplastic or is an elastomer-modified thermoplastic (thermoplastic elastomer). The term “core polymer” also comprehends, in particular, mixtures of polymers, known as polymer blends. Polymer blends modify properties of different polymers; in particular the reversibility of the deformation of thermoplastic polymers is increased through the addition of elastomers.

The core polymer has at 21° C. preferably a breaking strength of between 5 and 60 MPa, more preferably from 20 to 60 MPa, and preferably a breaking extension of between 2% and 1500%, more preferably between 300 and 700% (measured by ISO 527 tensile test).

If a high reversibility in the deformation of the fiber adhesive is desired, repetitions of the tensile test ought to achieve extension values in the range of 20% and 100%, preferably between 60% and 90% of the value measured in the 1st pass (recovery).

Suitable core polymers are polymers obtainable by free-radical polymerization (vinyl polymers), examples being polyolefins, such as polyethylene, polypropylene, polycondensates, examples being polyesters, polyamides, or polyadducts, examples being polyurethanes.

Preferred core polymers are polyamide, polyamide copolymers with more than 20% by weight polyamide fractions, polypropylene, polyesters, such as polyethylene terephthalate, styrene homopolymers or copolymers, especially styrene-butadiene copolymers.

Preference is given in particular to thermoplastic styrene copolymers. Styrene copolymers of this kind are composed preferably of 5% to 99% by weight, more preferably of 10% to 85% by weight, of styrene.

Particular preference is given to styrene-butadiene copolymers, especially block copolymers.

Preferred block copolymers are those with a hard/soft/hard block sequence or star block copolymers with external hard blocks and internal soft blocks.

The weight fraction of the soft blocks ought preferably to be greater than 50% by weight, more preferably 60% to 90% by weight, with particular preference 65% to 85% by weight.

The hard blocks are composed preferably of pure polystyrene or of copolymers of styrene with alpha-methylstyrene, ring-hydrogenated styrene, para-methylstyrene, paratertiary-butylstyrene or 1,1 diphenyl ether.

The soft phase is preferably composed of a polymer having a glass transition temperature of less than or equal to 20° C., in particular less than 0° C., more preferably less than −20° C. Soft phases may be composed of polybutadiene, polyisoprene or subsequently hydrogenated polymers thereof, polyisobutene, random copolymers of styrene and butadiene, styrene and isoprene, and also combinations of these monomers.

The soft phase may also have a monomer composition which changes along the polymer chain.

The block sequences (hard/soft)_(n+1) and (hard/soft/hard)_(n+1) are also provided by the invention, with n being a natural number (n=0, 1, 2, . . . ).

Particular preference is given to linear styrene-butadiene block copolymers of the general structure S-(S/B)-S with one or more (S/B) random blocks located between the two S blocks and having a static styrene/butadiene distribution. Block copolymers of this kind are obtainable by anionic polymerization in a nonpolar solvent with the addition of a polar cosolvent or a potassium salt, as described for example in WO 95/35335 or WO 97/40079.

The vinyl content is understood to be the relative fraction of 1,2 linkages of the diene units, relative to the total of the 1,2,1,4-cis, and 1,4-trans linkages. The 1,2-vinyl content of the styrene-butadiene copolymer block (S/B) is preferably below 20%, in particular in the range from 10% to 18%, with particular preference in the range of 12%-16%.

Particularly suitable styrene-butadiene copolymers are available for example under the trade name Styroflex® from BASF.

The nontacky core may comprise, besides the core polymer, further constituents, examples being additives such as stabilizers, e.g. UV stabilizers and heat stabilizers, fillers, pigments, e.g. TiO2, dye pigments, carbon black, carbon nanotubes, glass, carbon fibers, phyllosilicates, and dyes. With preference the core is composed of more than 90% by weight of the core polymer.

The Outer Shell

The outer shell is composed of a permanently tacky adhesive, i.e., of a pressure-sensitive adhesive. The adhesive in question is in particular a pressure-sensitive adhesive which can be detached again without residue when subsequently bonded.

In particular the adhesive in question is a permanently tacky hotmelt adhesive (i.e., an adhesive which is processed without water or solvent).

The adhesive has at 21° C. preferably a peel strength (as a measure of the adhesion) of at least 1 N/2.5 cm by the measurement method below. The peel strength is preferably 1 to 25, more preferably 2 to 20, very preferably 3 to 15 N/2.5 cm.

The shear strength, as a measure of the cohesion, is preferably greater than 10 minutes by the measurement method below. The shear strength is preferably 10 to 1440 minutes, more preferably 100 to 1000 minutes, very preferably 200 to 500 minutes.

The Test Methods

The adhesive is coated at 20 g/m2 (solids) onto polyethylene film and dried at 90° C. for 3 minutes.

For the determination of the peel strength (adhesion) a test strip 2.5 cm wide is adhered to a chromed V2A stainless steel test plate and rolled on once using a roller weighing 1 kg. It is then clamped by one end into the upper jaws of a tension/extension testing apparatus. The adhesive strip is removed from the test surface (V2A stainless steel) at 300 mm/min and at an angle of 180°; that is, the adhesive strip was bent around and removed parallel to the test plate, and the force required to accomplish this was measured. The measure of the peel strength is the force in N/2.5 cm which results as the average value from five measurements (corresponding to AFERA standard method). The peel strength is determined 24 hours after bonding.

For the determination of the shear strength the test strips are adhered with a bonded area of 25 mm² to a chromed V2A stainless steel test plate, rolled on once using a roller weighing 1 kg, stored for 10 minutes (under standard conditions, 1 bar, 21° C.), and then loaded in suspension with a 0.5 kg weight (under standard conditions, 1 bar, 21° C.). The measure of the shear strength was the time which elapsed before the weight fell down, in minutes; the average is calculated in each case from five measurements (corresponding to PSTC standard method).

The adhesive comprises as an essential constituent at least one binder and if appropriate further additives.

The binder may in particular be a synthetic polymer (referred to below as adhesive polymer). The term “adhesive polymer” is also intended to comprehend mixtures of different polymers; in particular, such mixtures may be mixtures of nontacky polymers with tacky polymers or with other polymers which produce tack in the mixture.

Suitable adhesive polymers include free-radically polymerized polymers, polyesters or polyadducts. Where the polymers concerned are not tacky, the above tack (peel strength) is set by addition of tackifying resins or plasticizers.

Mention may be made, for example, of styrene copolymers, especially styrene-butadiene copolymers, ethylene vinyl acetate copolymers, polyurethanes, polyisobutylenes, and noncrystalline polyolefins, comprising an appropriate amount of plasticizers or tackifying resins.

Also suitable in particular are block copolymers.

In the case of styrene block copolymers it is possible for example to use free-radically polymerized styrene-butadiene copolymers which have a butadiene content of greater than or equal to 50% by weight, preferably greater than or equal to 60% by weight, more preferably greater than or equal to 80% by weight.

Styrene-butadiene copolymers may for example be diblock, triblock or multiblock copolymers, whose soft-phase fraction is preferably greater than or equal to 60% by weight.

Preference is given in particular to diblock copolymers and to mixtures thereof with triblock, multiblock or star block copolymers.

Where plasticizers are used in the outer shell, plasticizers are used (i.e., blended with the polymers) which have little or no influence on the properties of the core polymer. Particular preference is given to combinations of a polar soft phase in the fiber core and a nonpolar soft phase in the tacky shell, or vice versa.

In particular the adhesive polymer is composed of free-radically polymerizable compounds (monomers). Preferably it is composed of at least 40% by weight, more preferably at least 60% by weight, and very preferably at least 80% by weight of what are called principal monomers.

The principal monomers are selected from C1-C20 alkyl (meth)acrylates, vinyl esters of carboxylic acids comprising up to 20 carbon atoms, vinylaromatics having up to 20 carbon atoms, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and 1 or 2 double bonds, or mixtures of these monomers.

Examples include (meth)acrylic acid alkyl esters having a C1-C10 alkyl radical, such as methyl methacrylate, methyl acrylate, n-butyl acrylate, ethyl acrylate, and 2-ethylhexyl acrylate.

In particular, mixtures of the (meth)acrylic acid alkyl esters are also suitable.

Vinyl esters of carboxylic acids having 1 to 20 carbon atoms are for example vinyl laurate, vinyl stearate, vinyl propionate, Versatic acid vinyl esters, and vinyl acetate.

Suitable vinylaromatic compounds include vinyltoluene a- and p-methylstyrene, a-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene, and, preferably, styrene. Examples of nitriles are acrylonitrile and methacrylonitrile.

The vinyl halides are ethylenically unsaturated compounds substituted by chlorine, fluorine or bromine, preferably vinyl chloride and vinylidene chloride.

Examples of vinyl ethers include vinyl methyl ether or vinyl isobutyl ether. Preference is given to vinyl ethers of alcohols comprising 1 to 4 carbon atoms.

As hydrocarbons having 2 to 8 carbon atoms and one or two olefinic double bonds mention may be made of butadiene, isoprene and chloroprene, ethylene or propylene. Polymers or copolymers obtained from butadiene or isoprene can also be hydrogenated subsequently.

Preferred principal monomers are the C1 to C10 alkyl acrylates and methacrylates, especially C1 to C8 alkyl acrylates and methacrylates, the acrylates in each case being particularly preferred.

Very particular preference is given to methyl acrylate, ethyl acrylate, n-butyl acrylate, n-hexyl acrylate, octyl acrylate, and 2-ethylhexyl acrylate, and mixtures of these monomers.

Besides the principal monomers the polymer may comprise further monomers, e.g., monomers having carboxylic acid, sulfonic acid or phosphonic acid groups. Carboxylic acid groups are preferred. Mention may be made, for example, of acrylic acid, methacrylic acid, itaconic acid, maleic acid or fumaric acid.

Further monomers include, for example, monomers comprising hydroxyl groups, especially C1-C10 hydroxyalkyl (meth)acrylates, (meth)acrylamide, and monomers comprising ureido groups, such as ureido (meth)acrylates.

Further monomers that may be mentioned include, moreover, phenyloxyethyl glycol mono(meth)acrylate, glycidyl acrylate, glycidyl methacrylate, and amino (meth)acrylates such as 2-aminoethyl (meth)acrylate.

Monomers which in addition to the double bond carry further functional groups as well, e.g., isocyanato-, amino-, hydroxy-, amide- or glycidyl-, may have the effect, for example, of improving the substrate adhesion. Those suitable include, in particular, cyclic lactams such as N-vinylpyrrolidone or N-vinylcaprolactam.

The polymer is preferably synthesized from at least 40% by weight, preferably at least 60% by weight and very preferably at least 80% by weight of C1-C20 alkyl (meth)acrylates, especially the abovementioned alkyl (meth)acrylates, and is therefore a polyacrylate.

Particular preference is given to adhesives whose adhesive properties can be adjusted by photochemical crosslinking, such as by irradiation with electron beams or UV light.

Preferably the adhesive polymer, therefore, is a polymer which is crosslinkable by irradiation with high-energy light, such as electron beams or, preferably, UV light.

The polymer is crosslinkable thus if, for example, hydrogen protons can be detached from the main chain of the polymer photochemically, including in particular through the use of a photoinitiator or as a result of electron beams, thereby producing a free radical which is able to enter into further chemical reactions.

The adhesive preferably comprises a photoinitiator.

The photoinitiator may comprise, for example, what are called α-splitters—that is, photoinitiators in which a chemical bond is cleaved to form 2 free radicals which initiate the further crosslinking or polymerization reactions.

Examples that may be mentioned include acylphosphine oxides (Lucirin® products from BASF), hydroxyalkylphenones (e.g., Irgacure® 184), benzoin derivatives, benzil derivatives, and dialkyloxyacetophenones.

In particular the compounds in question may be what are called H abstractors, which detach a hydrogen atom from the polymer chain; these are, for example, photoinitiators having a carbonyl group. This carbonyl group is inserted into a C—H bond to form a C—C—O—H moiety.

Here mention may be made in particular of acetophenone, benzophenone, and derivatives thereof.

It is possible to use both classes of photoinitiators alone or else in a mixture. Preference as photoinitiator is given to H abstractors.

In one particular embodiment the photoinitiator or at least one of the photoinitiators, if a mixture is used, is attached to the adhesive polymer.

With particular preference the photoinitiator in question is a photoinitiator which is incorporated into the polymer chain by means of free-radical copolymerization. For that purpose the photoinitiator preferably comprises an acrylic or (meth)acrylic group.

Suitable copolymerizable photoinitiators are acetophenone derivatives or benzophenone derivatives which comprise at least one, preferably one, ethylenically unsaturated group. The ethylenically unsaturated group is preferably an acrylic or methacrylic group.

The ethylenically unsaturated group may be attached directly to the phenyl ring of the acetophenone derivative or benzophenone derivative. In general there is a spacer group located between the phenyl ring and ethylenically unsaturated group.

The spacer group may comprise, for example, up to 100 carbon atoms.

Suitable acetophenone derivatives or benzophenone derivatives are described, for example, in EP-A-346 734, EP-A-377199 (1st claim), DE-A-40 37 079 (1st claim) and DE-A-38 44 444 (1st claim) and by this reference are also disclosed in the present application. Preferred acetophenone derivatives and benzophenone derivatives are those of the formula

in which R¹ stands for an organic radical having up to 30 carbon atoms, R² for a hydrogen atom or a methyl group, and R³ for an optionally substituted phenyl group or a C1-C4 alkyl group.

R¹ stands with particular preference for an alkylene group, especially for a C2-C8 alkylene group.

R³ stands with particular preference for a methyl group or a phenyl group.

The adhesive comprises preferably 0.0001 to 0.5 mol, more preferably 0.0002 to 0.1, very preferably 0.003 to 0.01 mol of the photoinitiator, or of the molecule group which is active as a photoinitiator and is attached to the polymer, per 100 g of adhesive polymer.

Suitable UV-crosslinkable polymers are available under the trade name acResin® from BASF.

The glass transition temperature (Tg) of the adhesive polymer is preferably −65 to +10° C., more preferably −65 to 0° C., very preferably −65 to −10° C., or −65 to −20° C.; in one very particularly preferred embodiment the glass transition temperature is −55 to −30° C., or −50 to −40° C.

The glass transition temperature of the polymer can be determined in accordance with customary methods such as differential thermoanalysis or differential scanning calorimetry (see, e.g., ASTM 3418/82, midpoint temperature).

Adhesive polymers can be prepared by copolymerizing the monomer with the use of the customary polymerization initiators and also, if appropriate, of regulators, polymerization taking place at the customary temperatures in bulk, in emulsion, e.g., in water or liquid hydrocarbons, or in solution. The polymerization, bulk polymerization for example, may also be carried out in an extruder.

The polymers are preferably prepared by polymerizing the monomers in solvents (solution polymerization).

The adhesive is preferably a hotmelt adhesive which comprises water or other solvents, examples being those from the solution polymerization, only in small amounts if at all. Solvent from the solution polymerization is therefore preferably separated off.

The adhesive preferably comprises less than 5 parts by weight, in particular less than 2 parts or 1 part by weight of water and/or solvent per 100 parts by weight of adhesive polymer. With particular preference the adhesive is substantially free from water and other solvents.

The adhesive preferably comprises at least one photoinitiator (see above). Where the photoinitiator in question is not a photoinitiator attached to the adhesive polymer, the photoinitiator may be added to the adhesive polymer at any time.

Where the adhesive polymer comprises blends, a suitable blend component comprises what are referred to as tackifying resins (tackifiers).

Tackifiers are, for example, natural resins, such as rosins and their derivatives formed by disproportionation or isomerization, polymerization, dimerization or hydrogenation. They may be in their salt form (with monovalent or polyvalent counterions (cations), for example) or, preferably, in their esterified form. Alcohols which are used for the esterification may be monohydric or polyhydric. Examples are methanol, ethanediol, diethylene glycol, triethylene glycol, 1,2,3-propanethiol and pentaerythritol.

Preferred tackifiers are natural or chemically modified rosins. Rosins are composed predominantly of abietic acid or derivatives of abietic acid.

Further additives which may be added to the adhesive are, for example, antioxidants, fillers, dyes, flow control assistants.

The pressure-sensitive adhesive or hotmelt adhesive is composed in particular of more than 40% by weight, with particular preference of more than 60% by weight, and with very particular preference of more than 80% by weight of the adhesive polymer, which is intended to include also mixtures of different polymers (see above).

The Fiber Adhesive in General

The fiber adhesive has the form of a fiber. The tacky outer shell is attached to the core. The fiber as a whole is extensible over a wide temperature range without the outer shell becoming detached.

The polymeric fiber preferably has a diameter of 8 μm to 500 μm, more preferably of 10 to 100 μm.

The diameter of the core is preferably 5 μm to 497 μm, more preferably 7 to 97 μm.

The layer thickness of the outer shell is preferably 3 μm to 100 μm, more preferably 5 to 50 μm, with very particular preference 10 to 40 μm.

The length of the fiber is arbitrary. The fiber may be in the form of short fiber sections, with a length for example of 0.1 mm to 10 cm; alternatively the fiber may be in the form of continuous fiber, depending on the desired further processing.

Production Processes

The fiber can be produced by standard methods. For example, the core material can be extruded as a fiber and the outer shell can be applied by customary coating techniques.

One particularly simple process is the coextrusion of core material and of the material for the outer shell.

For the coextrusion the core material is heated preferably to temperatures between 150 and 300° C. and the material of the outer shell to temperatures between 80 and 200° C., more preferably between 100 and 180° C., very preferably between 110 and 150° C., and then coextrusion too takes place at these temperatures.

The polymeric fiber obtained is preferably drawn additionally immediately after coextrusion, the length increase (degree of drawing) being preferably 10% and 300%, more preferably 50% to 250%, and with very particular preference 100% to 200%.

Where the adhesive polymer is a UV-crosslinkable polymer, production of the fiber is followed by irradiation with high-energy light, in order thus to set the desired adhesive properties.

The radiation energy can amount for example to 10 mJ/cm2 to 1500 mJ/cm2 of irradiated area.

Use of the Polymeric Fiber

The polymeric fiber can be used for any of a very wide variety of purposes. Through combination of the adhesive properties with the mechanical properties of a fiber, in particular a high elasticity and breaking strength, entirely new applications are opened up.

The fiber may be used, for example, for producing a sheetlike structure, a nonwoven fiber web for example. Sheetlike structures of this kind are tacky but at the same time also possess a high elasticity. The sheetlike structures can be designed in terms of their dimensions and distances between the fibers in such a way that they are permeable, for example, to gases and liquids, while unwanted constituents are retained and cling to the fibers.

The polymeric fiber is therefore suitable for producing permeable layers, examples being permeable nonwovens, membranes, or other textile sheetlike structures, which can be used as filters for separating off any of a very wide variety of materials. Suitable examples include dust filters or other sheetlike structures for removing dispersed or emulsified particles from liquid or gaseous media.

In addition the polymeric fiber can be used for binding, immobilizing or strengthening small piece goods. Sheetlike structures made from the fiber can be used for transporting or sorting goods. 

1. A tacky polymeric fiber having a nontacky core and a tacky outer shell.
 2. The tacky polymeric fiber according to claim 1, wherein the core comprises a polymer (referred to below for short as core polymer) which at 21° C. has a breaking strength of between 5 and 60 Mpa and a breaking extension of between 2% and 1500% (by ISO 527 tensile test).
 3. The tacky polymeric fiber according to claim 1, wherein the core polymer is a thermoplastic or is a thermoplastic which has been modified with an elastomer (thermoplastic elastomer).
 4. The tacky polymeric fiber according to claim 1, wherein the core polymer is a styrene copolymer.
 5. The tacky polymeric fiber according to claim 1, wherein the core is composed of more than 90% by weight of the core polymer and may further comprise additives which may be stabilizers, fillers, carbon nanotubes, pigments, dyes.
 6. The tacky polymeric fiber according to claim 1, wherein the outer shell is composed of a permanently tacky adhesive, of a pressure-sensitive adhesive.
 7. The tacky polymeric fiber according to claim 6, wherein the adhesive is a permanently tacky hotmelt adhesive.
 8. The tacky polymeric fiber according to claim 1, wherein the adhesive, or hotmelt adhesive, at 21° C. has a peel strength of at least 1 N/2.5 cm.
 9. The tacky polymeric fiber according to claim 1, wherein the hotmelt adhesive comprises as binder a polymer (referred to below as adhesive polymer) which is obtainable by free-radical polymerization of ethylenically unsaturated compounds.
 10. The tacky polymeric fiber according to claim 9, wherein the adhesive polymer is a UV-crosslinkable polymer.
 11. The tacky polymeric fiber according to claim 1, wherein the adhesive is composed of more than 40% by weight of the adhesive polymer and if appropriate may further comprise additives, which may be tackifying resins, plasticizers, UV blockers, antioxidants, dyes or fillers.
 12. The tacky polymeric fiber according to claim 1, wherein the polymeric fiber has a diameter of 8 μm to 500 μm.
 13. The tacky polymeric fiber according to claim 12, wherein the core has a diameter of 5 μm to 497 μm.
 14. The tacky polymeric fiber according to claim 13, wherein the outer shell has a layer thickness of 3 μm to 100 μm.
 15. A process for producing the tacky polymeric fiber according to claim 1, which comprises coextruding the core material and the material for the outer shell.
 16. The process according to claim 15, wherein the core material is heated to temperatures between 150 and 300° C. and the material of the outer shell is heated to temperatures between 80 and 200° C. and the coextrusion takes place at these temperatures.
 17. The process according to claim 16, wherein the polymeric fiber obtained is additionally drawn immediately after coextrusion, the length increase (degree of drawing) being preferably 10% to 300%.
 18. The method of using the polymeric fiber according to claim 1 for producing permeable layers, which may be permeable nonwovens, membranes, textile sheet structures.
 19. The method of using the polymeric fiber according to claim 17 for binding, immobilizing or strengthening small piece goods.
 20. The method of using the polymeric fiber according to claim 17 for producing gas-permeable sheet structures with a large adhesion area, for producing dust filters or particle filters for example.
 21. The method of using the polymeric fiber according to claim 17 for producing elastic sheet structures which at application temperature are tacky and/or detachable, for transporting and/or sorting goods.
 22. The method of using the polymeric fiber according to claim 17 for producing permeable sheet structures with a large adhesion area for removing dispersed or emulsified particles from liquid or gaseous media. 